1880.] 4dO [Hall. 



14. The continual reciprocal action, between attracting centres, ■ g oc '— 



of disturbances proportional to mass. 



15. The limiting influence of parabolic velocities, upon tendencies to dis- 

 sociation and to aggregation. 



10. The ratio of stress-opposing force, at Laplace's limit, to parabolic 



— = land to orbital (*) velocity. 

 i/2/ 



17. The influence of centres of linear and of spherical oscillation. 



18. The conjoint influence of centres of nucleation, of density, of nebu- 

 losity, of rotary inertia, and of reciprocity. 



19. The equations of relation between oscillatory and orbital motion. 



20. The interesting and suggestive fact, important in chemistry and gen- 

 eral physics as well as in astronomy, that the central stress-opposing value in 



the solar system H tt J is the velocity of light. 



The Relations of the- Crystalline Rocks of Eastern Pennsylvania to the Silu- 

 rian Limestones and the Hudson River Age of the Hydromke Schists. 

 By Charles E. Hall. With a Plate. 



{Read before the American Philosophical Society, January 2, 1880.) 

 Recently Prof. Frazer called the attention of the Academy of Natural 

 Sciences to the fact of the occurrence of the fossil Buthotrephis flexuosa in 

 the Peach Bottom roofing slates of York county, Pennsylvania. As Prof. 

 Lesquereus admits that this fossil does not extend below the Trenton lime- 

 stone, it is in all probability within the Hudson river group. Dr. Emmons 

 assigned this fossil to the Taconic System. Since Dr. Emmons' time, I 

 think the fossiliferous bed of the Taconic system have been pretty well 

 proven to be of the Cambrian series, which would place this Taconic fossil 

 of Emmons somewhere about the Hudson river group. 



I embrace this opportunity to state some facts from which I have drawn 

 conclusions concerning the relative positions of the rocks forming the crys- 

 talline series of Eastern Pennsylvania. 



I shall endeavor to make my statements concise, and I think my reason- 

 ing will be understood. 



We have the following series of rocks: 



First. A series of granitoid, syenitic, quartzose, and micaceous schistose 

 rocks, to be seen on the Delaware river above the city bridge at Trenton, 

 and extending in a south-easterly belt across Bucks and Montgomery coun- 

 ties, as far west as Chestnut Hill, Philadelphia. 



Second. A series of syenitic, hornblendic and quartzose rocks extending 

 from the neighborhood of Chestnut Hill westward across the Schuylkill 

 river, and covering a greater part of the northern portion of Delaware 

 county. Fine exposures of this rock are to be seen on the Schuylkill river 

 below Spring Mill, Montgomery county. This series may be the upper 

 members of the first, or that extending from the Delaware river to Chest- 

 nut Hill. 



